Macros are ugly, and multi-line macros are even more ugly.  This patch
converts processDecodeInput() to a C function.  The disadvantage may
be that the function does not have access to the caller's local
variables, which might be regarded as an advantage on the other hand.
For this reason, we have to pass variable references.  This costs a
tiny bit of performance, but it's worth eliminating this monster
macro, and further patches will optimize this cost down.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7254 09075e82-0dd4-0310-85a5-a0d7c8717e4f